1. Technical Field
This invention relates to an improved steam supply system for supplying steam to cyclic high demand steam loads. More particularly, it relates to a steam supply system for use in food processing steam applications.
2. Background Art
It is sufficient to say that steam boilers, generators and power systems have been in use for a long time and in a wide range of applications. Closed loop systems, wherein the expended steam is condensed and returned to the boiler, are used to generate electricity, to heat buildings, and even to propel nuclear powered submarines through the depths of the oceans. Open looped systems where the expended steam is discharged to the atmosphere have far fewer applications, and are generally used only in situations where the steam becomes contaminated during its use. One such application is the use of steam to peel bulk quantities of food products. The concept behind steam peeling is to place the food product into a vessel of some sort, rapidly introduce large quantities of steam for between 10 and 15 seconds in order to cook the surface skin of the food product without cooking the core or pulp of the food product.
There are a number of food products which can be steam peeled in this manner, they include, amongst others, tomatoes, cucumbers, carrots, beets, onions and potatoes. For purposes of this specification, steam peeling of potatoes will be used as an example, however the technology and information provided also applies to a wide variety of other food processing applications.
Potato processing is a rapidly growing and developing industry. In general terms, potatoes are harvested from the ground and stored, in bulk, in storages wherein temperature and humidity are closely controlled in order to maintain the potatoes in as close to original harvested condition as possible. Obviously the potatoes are still sheathed in protective skins. The first step in processing these stored potatoes into frozen french fries, hash browns, potatoes or the like, usually involves washing the whole potatoes to remove entrained dirt. After the potatoes are initially rinsed or washed, they are dropped, in bulk, into a peeling vat, which is a pressure vessel, having a large opening at the top. Once the vat has been filled with potatoes, the opening is sealed and saturated steam, usually at a temperature around 400.degree. F. is injected into the vat peeler to rapidly cook the outer surfaces of the potato. Typically it takes approximately 0.08 pounds of steam at 400.degree. F. per pound of potato for 15 seconds to impart a sufficient amount of heat to the surface of the potatoes to cook the skin without cooking the potato itself.
Once the potatoes have been cooked in the vat peeler for 15 to 20 seconds, the spent or dead steam is released to atmosphere, the vat peeler opened, and the potatoes dumped into some sort of a brushing device, or other apparatus where the cooked skins are separated from the potatoes. Then the peeled potatoes, in the typical processing operation, are again washed, cut into the desired pieces and further processed to produce the desired final product.
It is important to cook only the skin of the potato, and minimize cooking the potato pulp. Potatoes are expensive to grow and the inadvertent cooking of a few extra millimeters of the potato pulp, can result in a significant loss of product in a large processing operation and contribute to increased waste treatment load. As a result, it is desirable to introduce the steam into the batch peeler as quickly as possible, to hold it in the peeler for the precise, empirically determined, amount of time, and then to quickly expunge it from the peeler in order to only cook the surface peel of the potatoes.
For a single 900 pound potato peeler, what this means is supplying about 12,000 pounds per hour to 20,000 pounds per hour of steam at 400.degree. F. and at approximately 250 p.s.i.a. for between 15 and 30 second periodic bursts and then drop to an effective zero demand for 50 to 120 seconds. Conventional packaged steam boilers are not designed to handle this cyclic steam demand.
The conventional steam boiler or steam generator uses fossil fuels, usually gas or oil, to boil water to make steam. They are not suitable for sustained cyclic operation with fast reaction times necessary to increase output 25% to 40% for 15 to 30 seconds and then reduce output by the same 25% to 40%. As a result, steam accumulators are sometimes used to store steam energy for use in the steam bursts needed for steam peeling processes. However, in the prior art, the designers of these steam supply systems for steam peelers have approached the problem and the use of the steam accumulator incorrectly which resulted in inefficient boiler operation and excessive peel loss. First, the accumulators have been connected into the steam supply systems as auxiliary sources of steam for the peelers. That is to say there is a direct piping connection between the steam boiler discharge line and the steam peeler to which the accumulator is also attached. Thus, in this conventional piping arrangement, when a burst of steam is introduced into the peeler, pressure in the boiler discharge line is rapidly decreased. Boiler controls respond by increasing firing rate and reducing feed water flow and to higher liquid levels in the boiler. These controls are normally slow reacting to preclude operation of the boiler outside its normal pressure and level band. In addition, when the boiler discharge pressure is rapidly drawn down as such, boiler pressure and steam accumulator pressure equalize for a period of time, which results in an inability to add or inject energy from the boiler into the accumulator, thus limiting the entire system's energy recharge time. This results in the requirement for oversized boilers in order to minimize the system recharge time so that rapid cycling or bursting of steam is possible. An example of such an accumulator design can be seen in HELLBORG, U.S. Pat. No. 1,896,308.
There are two types of accumulators which can be generally classified as dry accumulators and wet accumulators. A dry accumulator is merely a large pressurized vessel which holds only steam. Dry accumulators have limited applications and generally are not in use today because of their size and inefficiencies. The preferred accumulator design is the wet accumulator wherein steam is introduced into a much smaller pressurized vessel, and is condensed and held as saturated liquid at an elevated pressure and temperature. Then when steam demand draws down the pressure in the steam system, the heated accumulator water becomes super-heated in relation to the lowered pressure within the accumulator pressure vessel, and as a result flashes to steam and is delivered through the steam system to the steam load. An example can be seen in FOHL, U.S. Pat. No. 1,867,143, and in prior art FIG. 1 of this specification.
In prior art systems, boiler steam is injected directly into the accumulator water through steam spargers. The boiler steam passes through the distribution pipes and is sparged into the accumulator water where the thermal energy from the boiler steam is transferred to the water and the boiler steam is condensed. Typically the accumulator and the boiler steam discharge line are both interconnected to the same steam load, and the accumulator acts as an auxiliary source of steam when the load draws down the pressure in the common discharge line. When the accumulator is thus in use as an auxiliary steam source, both the boiler pressure and accumulator pressure are the same, thus no steam will flow from the boiler discharge into the accumulator.
In these prior art systems, as steam demand is reduced boiler discharge line pressure will again increase, and saturated water in the accumulator will stop flashing and the boiler will once again be able to commence recharging the accumulator. Thus there is an inherent time period during times of high steam demand and for a period of time thereafter when the boiler cannot recharge the accumulator. This time lag can be significant, and in practice has been found to be the limiting factor in some boiler and steam supply system design for food product steam peelers.
Another problem with conventional designs where the mechanism for the transfer of energy from the boiler steam to the accumulator water is sparging the gaseous steam through the accumulator water is that during draw down caused by high demand the sparging rate of boiler steam through accumulator condensate dramatically increases to the point where a gaseous path is created between the sparging point and the accumulator steam discharge point. In effect, the boiler steam forms its own pathway directly through the accumulator water, this reduces the rate of transfer of energy from the boiler steam to the accumulator water. In practice it has been found that this results in a poor distribution of injected thermal energy within the accumulator water to the point where there are cold and hot spots. This results in an additional increase in accumulator recharge time since convection flow must be reestablished within the accumulator for good mixing.
If the steam load is averaged over time, and it has a slow changing average load, as it is in food processing applications, then it would be better to have a steam supply system wherein the boiler is isolated from the pressure draw down resulting from cyclic load so that the boiler can operate in a steady state configuration. This would result in improved operation of the boiler and a more equal matching of boiler capacity to total average steam load, thus eliminating the need of oversized boilers.
Accordingly, it is an object of this invention to provide an open loop steam supply system wherein there is effective, but de facto, isolation of the boiler discharge line from the steam load. It is another object of this invention to provide an accumulator wherein the energy from the boiler steam is transferred to the accumulator water through conductive heat transfer surfaces of combination heat exchanger and sparge pipes as opposed to direct contact condensation of sparging boiler steam, thus eliminating the formation of gaseous pathways through the accumulator water during periods of high demand.